In recent years, in order to achieve energy saving, a motor of an air conditioner and a main motor of an electric vehicle and the like have been required to reduce power consumption. These motors are often used by being rotated at high speed. Accordingly, a non-oriented electrical steel sheet used for an iron core of the motor has been required to improve core loss and enhance strength in a frequency region higher than 50 Hz to 60 Hz being a commercial frequency. The enhancement in strength has been required to prevent deformation and breakage of the steel sheet during the rotation at high speed.
For the improvement of core loss of the non-oriented electrical steel sheet in the high frequency region, an increase in electrical resistance due to an increase in a content of Si or Al and a reduction in thickness of the non-oriented electrical steel sheet itself have been known to be effective.
However, when the content of Si or Al is increased, brittleness is significantly deteriorated. For this reason, abnormalities during operation such as a fracture of steel sheet and the like frequently occur at the time of manufacture, resulting in that productivity is significantly reduced and cost is significantly increased. Further, when the non-oriented electrical steel sheet is thinned, it becomes difficult to secure the strength, resulting in that the steel sheet may be largely deformed during the rotation at high speed.
Further, it has also been studied to increase the electrical resistance by adding Cr in order to improve the core loss of the non-oriented electrical steel sheet in the high frequency region.
However, when a non-oriented electrical steel sheet containing Cr is manufactured through the same method as that of a non-oriented electrical steel sheet containing no Cr, an amount of dissolved nitrogen in a molten steel is increased, resulting in that a large amount of fine AlN inclusions are likely to precipitate at the time of annealing. As a result of this, growth of crystal grains is inhibited due to a pinning effect, and the crystal grains become fine. As a result of this, it is not possible to sufficiently improve the core loss even when the electrical resistance is increased.
This is because a nitrogen solubility in a molten steel containing Cr is higher than a nitrogen solubility in a molten steel containing no Cr. For example, a nitrogen solubility in a molten steel containing Cr of about 5 mass % is higher by several tens of percent than that of a molten steel containing no Cr.
In order to suppress the increase in the amount of dissolved nitrogen, it can be considered to prevent the contact between the air and the molten steel. However, although a countermeasure to prevent the contact between the molten steel and the air is taken also in the manufacture of the non-oriented electrical steel sheet containing no Cr, it is difficult to completely prevent the contact. It is possible to further inhibit the contact by improving a manufacturing facility and a manufacturing method of the non-oriented electrical steel sheet containing no Cr and enhancing adjustment of atmosphere and the like, but, a large cost is required to sufficiently inhibit the contact. Further, it can also be considered to lower an annealing temperature to suppress the precipitation of fine AlN inclusions, but, the necessity to perform annealing for a long period of time arises, which leads to a decrease in productivity and an increase in cost.